1. Field of the Invention
The present invention relates to a bulk DC/DC converter, and more particularly, to a bulk DC/DC converter in constant on-time mode.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional bulk DC/DC converter 100. The bulk DC/DC converter 100 converts an input voltage source VIN to be an output voltage source VOUT, wherein the voltage VOUT is lower than the voltage VIN. As shown in FIG. 1, the DC/DC converter 100 comprises a control circuit 110, a switch set 120, an inductor L, an output capacitor COUT, and a voltage-dividing set 130. The switch set 120 comprises two switches Q1 and Q2. The voltage-dividing set 130 comprises two voltage-dividing resistors RB1 and RB2. The control circuit 110 comprises a comparator CMP1, a pulse generator 111, and a drive circuit 112. The operation principles of the bulk DC/DC converter 100 are described as follows.
The control circuit 110 controls the operation of the DC/DC converter 100 by constant on-time manner. That is, when the control circuit 110 detects the output voltage VOUT is lower than a predetermined value, the switch Q1 is turned on for a constant period of time (constant on-time) by the control circuit 110 (while the switch Q2 is turned off) for allowing the input voltage source VIN conducting to the inductor L through the switch set 120.
During the operation of the DC/DC converter 100, the inductor L carries current IL, and the current IL flows into the equivalent serial resistor RE of the output capacitor COUT so that the resistor RE carries voltage VL reflecting the current IL. As shown in FIG. 1, the waveform of the voltage VL is saw-toothed because the switch Q1 is periodically turned on/off. The comparator CMP1 receives the feedback voltage VFB divided from the voltage VL and the output voltage VOUT by the resistors RB1 and RB2, and compares with a reference voltage VREF1, so as to determine when to turn on the switch Q1. More specifically, when the voltage (feedback voltage VFB) on the negative input end of the comparator CMP1 is lower than the voltage (reference voltage VREF1) on the positive input end of the comparator CMP1, which means the output voltage VOUT is too low, and the switch Q1 is needed to be turned on for allowing the input voltage source VIN to charge the inductor L and the output capacitor COUT, the comparator CMP1 controls the pulse generator 111 to generate a pulse signal PON. When the pulse generator 111 is triggered by the comparator CMP1, the pulse generator 111 generates a pulse signal PON with a predetermined duration TP and predetermined logic. The drive circuit 112 controls the switch set 120 according to the pulse signal PON. More particularly, when the drive circuit 112 receives the pulse signal PON, the switch Q1 is driven to turn on for the predetermined duration TP. In addition, except in the dead time both of the switches Q1 and Q2 are turned off, when the switch Q1 is turned on, the switch Q2 is turned off; when the switch Q1 is turned off, the switch Q2 is turned on. In this way, the control circuit 110 controls the DC/DC converter 100 to operate regularly in constant on-time mode.
However, not all kinds of capacitors definitely have equivalent serial resistors, and because of the improvement to the manufacture of capacitors, the equivalent serial resistances of the capacitors become smaller, or even do not exist. For example, the multi-layer ceramic capacitor (MLCC) is very similar to an ideal capacitor and therefore the equivalent serial resistor does not exist on the MLCC. Consequently, when the MLCC is utilized as the output capacitor COUT, the resistor RE does not exist, and thus the information of the current IL cannot be informed to the control circuit 110, causing the control circuit 110 unable to control the DC/DC converter 100 according to the voltage VL effectively. The control circuit 110 is still able to operate by the feedback of the output voltage VOUT. However, the phase of the output voltage VOUT is far behind the phase of the voltage VL because of the output capacitor COUT, which makes the control circuit 110 unable to react to the variation of the output voltage VOUT in time. For this reason, the DC/DC converter 100 is not able to operate stably in constant on-time mode while utilizing the MLCC as the output capacitor, causing inconvenience.